In the description of a technical system, for example a power station, reference is made to various parameters such as, for example, pressures, mass flows, etc. The parameters obey certain physical laws, such as for example those of mass or energy balances, which can be expressed by a system of equations. The solutions of this system of equations are the state variables of the technical system. From these state variables it is possible in turn to calculate diagnostic variables relevant for the operation of the technical system, such as for example the efficiency of a power station. The specific state of a technical system can, furthermore, be sensed by measurements. The measured variables of the measurements could directly reflect the value of a state variable; but it is also possible for measured variables which are derived from the state variables to be measured. For example, it is possible to measure the temperature of a technical system whereas the actual system state variables are the enthalpy and the pressure. In order to determine the state variables from the measured variables, one generally carries out a measurement and looks for the values of the state variables which solve the system of equations, and for which the derived measured variables lie closest to the measured values determined by the measurement (see for example VDI Guideline 2048).
The problem can arise, due to the number of equations in the system of equations being too small, or too small a number of measurement points, that individual state variables or individual diagnostic variables remain indeterminate. In addition, the state variables or diagnostic variables, as applicable, may be subject to great uncertainty, because of measurement errors. It is therefore necessary to decide which measurements would permit the precision of certain state variables to be improved, or would permit any determination at all of certain state variables. For this purpose, it is usual to fall back on the advice of experienced engineers, and the suggestions of these engineers can be checked by simulation programs. However, this requires time-consuming analyses.